The present disclosure is in the field of ripple-current reduction techniques and, more particularly, relates to the application of such techniques to power electronic circuits, particularly those of AC converters that include inductors.
Inductors are used in many ways in power electronic converters including operation as filters, energy storage and high frequency decoupling. In most cases a low frequency current and a high frequency ripple current will flow in the main inductor. This current is present due to the switching involved in the operation of power electronic circuits. An inductor may also be connected to a capacitor to create a low-pass filter to allow the flow of low frequency current and to reduce AC ripple of the desired voltage. A critical problem that arises in such circuitry is that ripple currents in a capacitor induce heating by reason of conductor losses and dielectric losses. The heating of the capacitor in turn reduces its life expectancy. Accordingly, any means that will reduce the ripple current into the capacitor has the potential to increase the life expectancy of a system that uses the capacitor. In addition, the reduction in the ripple current can reduce the required total capacitance which in turn can lead to a reduction in the size of the capacitor and, hence, of the system. This is conventionally achieved by the mechanism of defining a fixed allowable ripple voltage across the terminals of the main capacitor before and after the ripple current reduction. An alternative embodiment can be achieved by reducing the inductance value of the inductor and maintaining the capacitance as per the original design.
The existence of old techniques, or techniques that have become available recently, can reduce the ripple voltage on a capacitor and may include an increase in the frequency of the ripple current. Unfortunately, this can also increase the stress on the capacitor more than the benefits provided by a reduction in the ripple current amplitude. This consequence follows because the losses in the capacitor are frequency dependent. Also, the problem is exacerbated when the power level of the converters is high. Another method has been used is to reduce the ripple voltage across the capacitor terminals by the addition of more filter components. However, since classic filter design requires that these filters carry the full power of the converter system, the cost of such additional filters outweighs the benefits. There is also difficulty in damping these complex filter arrangements. In addition, the total ripple can only be spread out between all the components.